The term "clay", as used in various areas of technology, is subject to wide variations in meaning. However, an inclusive definition normally would be a naturally occurring sedimentary material generally composed of hydrated silicates of aluminum, iron or magnesium and often containing hydrated alumina and iron impurities. The particles of a clay are typically of at least near-colloidal size in at least one dimension (platelets are typical) and commonly develop thixotropic flow properties when sufficiently pulverized and wetted.
The organization of clay types remained somewhat poor until the development of satisfactory x-ray techniques for studying the atomic structure of individual clays. A paper: Kaolin Materials, U.S. Geological Survey Professional Paper 165-E, C. S. Ross and P. F. Kerr, 1930, pp. 151 to 176, is widely recognized as the beginning of a systematic approach to clay mineralogy. The monograph "Crystal Structures of Clay Minerals and their x-ray Identification", edited by G. W. Brindley and G. Brown for the Mineralogical Society, is the most convenient standard reference. More recent work has been reviewed in a Mineralogical Society of American Monograph (Reviews in Mineralogy, Vol 16, "Hydrous Phyllosilicates, Ed S. W. Bailey, (1988)).
Included in the classes of clay minerals are smectite clays and kandite clays, the latter synonymously called diazeolites, serpentines, septochlorites and a variety of other specific names, depending on composition and layer orientation.
Smectites generally layered clays represented by the general formula: EQU (Si.sub.8).sup.iv (Al.sub.4).sup.viO.sub.20 (OH.sub.4)
where the IV designation indicates an ion coordinated to four other ions, and VI designates an ion coordinated to six other ions. The IV coordinated ion is commonly Si.sup.4+, Al.sup.3+, or Fe.sup.3+ but could also include several other four coordinated ions, e.g., P.sup.5+, B.sup.3+, Ge.sup.4+, Be.sup.2+, etc. The VI coordinated ion is typically Al.sup.3+ or Mg.sup.2+, but could also include many other possible hexacoordinate ions, e.g., Fe.sup.3+, Fe.sup.2+, Ni.sup.2+, Co.sup.2+, Li.sup.+, etc. The charge deficiencies created by substitutions into these cation positions are balanced by one or more cations located between the structured platelets. Water may be occluded between the layers and either bonded to the structure itself or to the cations as a hydration shell. Commercially available clays typical of this class include variants of montmorillonite, bentonite, and hectorite. The pillaring of said materials is well established and characterized (e.g., U.S. Pat. Nos. 4,176,090; 4,248,739; and 4,271,043) and the state of the art has recently been reviewed by Vaughan (Amer. Chem. Soc. Symp. Ser. #368, p. 308-323, (1988)), particularly as the basic concept has been applied to layer compounds other than clays.
Kandite clays, also often called "kaolinite" minerals, are made up of 1:1 layers of tetrahedrally oxygen coordinated silicon, bonded to layers of octahedrally bound cations. In kaolinite, dickite and nacrite all of the tetrahedral cations are Si.sup.4+ and all of the octahedral cations are Al.sup.3+ (so called dioctahedral forms). However, in the serpintinite varieties, major substitution of Al.sup.3+ and Fe.sup.3+ occurs for Si.sup.4+ in the tetrahedral layer and a range of di-and trivalent cations substitutes for Al.sup.3+ in the octahedral layer. The ion Mg.sup.2+ is typically substituted for Al.sup.3+, although any of the Fourth Period Transition elements, e.g., V, Cr, Mn, Fe, Co, Ni, Cu, Zn, may serve as substitutes. In some locations they may form major deposits, as in the case of garnierite, a major nickel ore. A main characteristic of the class is that each member generally has a 1:1 neutral layer. The ideal stoichiometry of the dioctahedral (kaolinite) and trioctahedral (chrysotile) end-members may be given respectively as: EQU Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4
and EQU Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4
Mixed layers are common, as they are in all clay mineral types. However, Kaolin is quite unique as a mineral in that it exists in very high purity deposits in many parts of the world. The deposits in the states of Georgia and North and South Carolinas are particularly famous. The single layer thickness of this repeating sheet is about 7.2 .ANG.. When layers of water separate the 1:1 sheets, the intersheet dimension expands to about 10.1 .ANG., as is seen in the halloysite variety of kaolinite. Halloysite in comparison is a relatively rare mineral in large deposits and rapidly loses water on exposure to air.
Sorption of various organic molecules, such as glycerol, have been reported for kaolinite and the 2:1 smectite clays. Organic molecules do not as a rule produce permanent pillaring between the clay layers, but form intercalates which may exhibit molecular sieve properties in some cases, as described by R. M. Barrer (Clays and Clay Minerals, v. 37, p. 385-95 (1989)) and Theng ("Formation and Properties of Clay Polymer Complexes, Elsevier Press" (1979)), but readily lose such properties on heating to moderate temperatures. Similarly, intercalation of organic salts, e.g., potassium acetate, has been reported and are reviewed by MacEwan and Wilson (ibid, p. 236) and Barrer (Zeolites and Clay Minerals p. 407, 1978). Permanent pillaring has not been reported in 1:1 kandite materials hitherto, and is the principal focus of a related invention (see copending patent application C-2441).
Recently several new layer structures have been successfully pillared with a variety of anionic, cationic and neutral inorganic polymeric molecules. They include various clays such as rectorite (European Patent Appln. 197,012) and tetrasilicia mica (Japanese Patent 56-142982); sheet silicic acids (European Patent Appln. 222,597; Deng et al, Chemistry of Materials, v. 1, p. 640-50, (1989)) which comprise a very large group of material (see F. Liebau for a review of such materials in "Structural Chemistry of Silicates" (Springer-Verlag (1985)); and zirconium phosphates (European Patent Appln. 159,756).
Several recent reviews of pillaring in clays and related sheet structures (Pinnavia, Science, 220, p. 365, (1983); Vaughan, "Catalysis Today", ibid 1988; Vaughan, in "Perspectives in Molecular Sieve Science", Ed. W. H. Flank et al, ACS Symp. Ser. 368, p. 308-23 (1988)) do not report kandite pillaring. Based on the viewpoint that pillaring requires either a change deficiency on the layer, or high sheet hydroxyl concentrations, the kandites would not be expected to be suitable pillaring substrates, as they are not recognized as having significant layer charge, and therefore have no ion exchange capacity. Reactivity and exchange in these materials is generally related to `OH` groups at the edges of the crystals. I have recently discovered that these can indeed be pillared using various methods to form porous materials (copending patent application C-2441).
Metakaolin is an ill defined material derived from kaolin by calcination at a temperature over about 550.degree. C. to dehydroxylate the sheet structure. Analogous kandites of other compositions undergo similar dehydroxylation. The precise temperature at which this transformation from ordered structure to disordered form depends to some degree on the chemistry and crystallinity of the particular kandite, in addition to the specific calcination conditions used, such as kind of atmosphere, partial water vapor pressure and rate of heating. The nature of these disordered materials, particularly metakaolin, has been reviewed by Grim (in "Clay Mineralogy", McGraw Hill (New York), p. 299-313 (1968)) and Hang and Brindley examined the changes in garnierite in detail (Clays and Clay Minerals, v. 24, p. 51 (1973)). More recently the general subject was reviewed by Brindley and Lamaitre (in "Chemistry of clays and clay minerals", Ed A. C. D. Newman, Min. Soc. (London) Monogr. 6, Ch. 7 (1987)). More detailed elucidation of these structures can be obtained from the recent application of .sup.27 Al and .sup.29 Si-MASNMR techniques (e.g., Fitzgerald et al, Solid State Ionics, v. 32, p. 378 (1989)). As even rehydration of metakaolin is difficult and only occurs in mildly treated forms, pillaring of these materials would be expected to be an unlikely process. I have discovered that, by carrying out the reaction in such a manner as to separate the disordered sheets, possibly by generating a charge deficiency in the metakandite by partial pre-leaching of the disordered solid, these materials can indeed be pillared and an ordered or partially ordered structure regenerated which has significant microporosity. The products of this invention are not limited to modified metakaolin, but include all "metakandite" compositions (i.e., those materials derived by dehydroxylation of kandites, and which have, in an X-ray sense, a disordered or random structure). A partial list of such subject kandites, for purposes of illustration, is given in Table 1.
TABLE 1 ______________________________________ EXAMPLES OF KANDITE MINERALS KAOLINS - SERPENTINES VI IV O.sub.5 (OH).sub.4 ______________________________________ Kaolin Al.sub.2 Si.sub.2 Halloysite Al.sub.2 Si.sub.2 Chrysotile Mg.sub.3 Si.sub.2 Garnierite Ni.sub.3 Si.sub.2 Amesite (Mg, Fe).sub.2 Al Si Al Cronstedtite (R.sub.3-x.sup.2+, Fe.sub.x.sup.2+) Si, Fe.sup.3+ Greenalite (Fe, Mg, Mn).sub.3 Si.sub.2 ______________________________________